1. Field
The present invention relates to an optical amplifier for use in a Wavelength Division Multiplexing (WDM) optical transmission system.
2. Description of the Related Art
With the wide spread of the Internet, image transmission services and so on, the amount of information to be transmitted over a network increases rapidly, and in order to support this, the introduction of a WDM optical transmission system has been underway. The WDM optical transmission system is started to adopt from long-distance trunk line systems and has also come to be adopted to a metropolitan ring network as a WDM optical transmission system having a wavelength routing function.
The optical amplifier for use in the WDM optical transmission system as described above is required in performance or characteristic to satisfy the requirements:    (a) To have a gain adjustable function in order to compensate a variety of optical losses;    (b) To prevent wavelength dependence of the gain and to prevent wavelength dependence of the gain even when the gain changes in order to transmit all optical signals contained in WDM light in a same manner; and    (c) To keep the gain caused by an optical amplifying catalyst since the gain wavelength characteristic changes if the gain caused by the optical amplifying catalyst is changed based on the amplification principle in an optical amplifier having an Erbium Doped Fiber (EDF), for example, as the optical amplifying catalyst.
FIG. 12 is a block diagram showing an example of a conventional optical amplifier configured to satisfy the requirements (a) to (c). In the conventional configuration example, the optical amplifier having the gain adjustable function is implemented by a combination of a fixed gain control section and an adjustable automatic optical loss control. More specifically, a variable optical Attenuator (VOA) 130 is deployed between the stages of the optical amplifying sections in the two-stage configuration having EDFs 101 and 102. The generated gain in the EDF 101 in the former stage is controlled by the gain control section 121 to be constant at a fixed target value, and the optical loss in the VOA 130 is controlled by the optical loss control section 140 to be constant at a variable target value. Furthermore, the generated gain in the EDF 102 in the latter stage is controlled by the gain control section 122 to be constant at a fixed target value. Thus, the gain variable optical amplifier satisfying the requirements (a) to (c) can be provided (refer to Japanese Laid-open Patent Publication No. 8-248455, for example).
The frequency of change of the number of wavelength of WDM light in the WDM optical transmission system having the wavelength routing function, like one having been adopted to metropolitan ring networks, is higher than that in a long distance trunk line system. When the number of wavelengths of WDM light is changed, the input strength of signal light of the optical amplifier changes. Also in this case, the characteristic that the generated gain in the optical amplifying catalyst does not change is required as the optical amplifier. In other words, the high speed characteristic for the automatic gain control by the optical amplifier (that is, the followability upon change of light input strength) is required.
In conventional optical amplifiers as shown in FIG. 12, an increase in speed of the automatic gain control in the entire optical amplifier including the optical loss in the VOA 130 is attempted by controlling pump light sources 111 and 112 by keeping the constant ratio (Pp1/Pp2) of the strengths Pp1 and Pp2 of pump light P1 and P2 to be supplied to the EDFs 101 and 102 in the stages independent of the change in gain set value of the entire optical amplifier. From the viewpoint of the high speed characteristic of the automatic gain control, providing a common gain control section 120 to the EDFs 101 and 102 as shown in FIG. 13, for example, is more advantageous than providing gain control sections 121 and 122 correspondingly and separately to the EDFs 101 and 102.
By the way, the conventional optical amplifier as described above must keep the generated gain in the EDFs 101 and 102 within a proper range for the reasons below:
(I) The optical transmission characteristic is adversely effected by a Noise Figure (NF) of the entire optical amplifier increased by an excessibly small gain generated in the EDF 101 in the first stage; and
(II) The optical transmission characteristic of signal light is adversely effected by the crosstalk (MPS-XT) between light SOUT, which has passed through the EDF once, and light XTOUT, which has passed through the EDF multiple times, since the Multi Pass Interference (MPI) caused within the optical amplifier is increased by an excessively large gain generated in the EDF 101 in the first stage, as shown in the conceptual diagram in FIG. 14.
However, in the conventional configuration, which is advantageous for increases in speed of the automatic gain control as shown in FIG. 13, the generated gains in the EDFs 101 and 102 are not directly monitored, and the automatic gain control over the entire optical amplifier is performed by assuming the constant ratio (Pp1/P2) of the pump light strengths of the EDFs. Thus, the generated gains in the EDFs 101 and 102 change when the gain set value for the entire optical amplifier change. Therefore, it is difficult to keep the generated gains in the EDFs 101 and 102 within a proper range, which has adverse effect on the transmission characteristic of the amplified signal light.
Here, the reasons (I) and (II) will be described in more details. First of all, for the configuration example in FIG. 13, parameters are defined as follows.    PSIN: Signal light input strength of an optical amplifier    NFAMP: Noise figure of an entire optical amplifier    MPIAMP: Amount of multi pass interference (MPI) occurring within an optical amplifier    PS1 and PS2: Signal light input strengths of the EDFs in the first and second stages    Pp1 and Pp2: Output strengths of the pump light sources in the first and second stages    G1 and G2: Generated gains of the EDFs in the first and second stages    NF1 and NF2: Noise figures of the EDFs in the first and second stages    Rb1 and Rb2: Return losses on the upstream sides of the EDFs in the first and second stages    Rf1 and Rf2: Return losses on the downstream sides of the EDFs in the first and second stages    L12: Optical Loss between the EDFs in the first and second stages    Generally, the noise figure NFAMP of an optical amplifier is expressed by:NFAMP=(NF1/PS1+NF2/PS2)×PSIN  [EQ1]
The signal input strength PS1 of the EDF in the first stage and the signal light input strength PS2 of the EDF in the second stage have the following relationship:PS2=PS1×G1/L12  [EQ2]
The EQ1 and EQ2 above describe that, as the generated gains G1 in the EDF in the first stage decreases, the signal light input strength PS2 of the EDF in the second stage decreases, and the noise figure NFAMP of the entire optical amplifier increases.
The amount of occurrence of multi pass interference MPIAMP within the optical amplifier as shown in FIG. 14 is dominant on the signal optical path near the EDF and can be expressed by:MPIAMP=G12/(Rb1×Rf1)+G22/(Rb1×Rf1)  [EQ3]
The EQ3 describes that, as the generated gain G1 in the EDF in the first stage increases, the amount of occurrence of the multi pass interference MPIAMP within the optical amplifier increases.
FIGS. 15 to 17 are examples of the specific comparison between a case where the ratio (Pp1/Pp2) of the pump light strength of each EDF is set high and a case where it is set low based on the descriptions above. Notably, the symbol GSET in the figures indicates the gain set value for the entire optical amplifier.
As shown in FIG. 15, in a case where the ratio (Pp1/Pp2) of the pump light strength is set high, the gain G1 of the EDF in the first stage increases as high as 25 dB by setting a relatively high value such as 30 dB as the gain set value GSET for the entire optical amplifier. This state corresponds to the area a having a high gain set value GSET on the horizontal axis in the relationship indicated by the solid line A in FIG. 17 and is the state relating to the reason (II), which is beyond the upper limit level (broken line) of the permissible range of MPI-XT. The top part of FIG. 17 shows the relationship of the ratio (Pp1/Pp2) of the pump light strengths about the gain set value GSET, and the bottom part of FIG. 17 shows the relationship of the generated gain G1 in the EDF in the first stage with the gain set value GSET.
As shown in FIG. 16, in a case where the ratio (Pp1/Pp2) of the pump light strength is set relatively low, the signal light input strength PS2 to the EDF in the second stage decreases as low as −20 dB by setting a relatively low value such as 15 dB as the gain set value GSET for the entire optical amplifier. This state corresponds to the area b having a low gain set value GSET on the horizontal axis in the relationship indicated by the solid line B in FIG. 17 and is the state relating to the reason (I), which is below the lower limit level (long dashed short dashed line) of the permissible range of NF deterioration.
Regarding the suppression of MPI-XT in an optical amplifier as described above, the International Publication Pamphlet No. 03/084007, for example, discloses the return loss on the output side is decreased according to the amount of attenuation of the interstage VOA, which can improve the MPI-XT. However, the conventional technology does not consider the relationship between the ratio (generated gain in the optical amplifying catalyst in each stage) given to the optical amplifying catalyst in each stage and the MPI-XT or NF deterioration, and it is difficult to solve the problem of keeping the generated gain in each EDF within a proper range, as described above.